As a rule, such spars consist of essentially two components that fulfill different functions. The so-called flanges absorb the normal forces from the flexural moments. The shear web that joins the flanges absorbs the lateral forces and conducts the shear between the flanges.
In the case of aircraft with a plastic construction, in which the spars are made of a fiber composite material, the flanges are made of a unidirectional layer (abbreviated UD layer). The shear web consists of a woven fabric or of a laid scrim at, for example, 45°, e.g. the so-called Balanced Ply Laminate 45. Unidirectional layer is the designation for a layer of a fiber composite in which all of the fibers are oriented in one single direction and it is thus also referred to as a 0° fiber laid scrim, that is to say, the fibers are aligned in the lengthwise direction of the tape. Here, the fibers are assumed to be ideally parallel and distributed homogeneously. The unidirectional layer is transversally isotropic.
All fiber prepregs, for example, fabrics, nonwovens, multiaxial laid scrims, used in fiber composites can be structured from unidirectional layers.
A woven fabric is a crosswise composite structure that is made up of two unidirectional layers that are positioned at an angle of 90° with respect to each other.
In wing spars as fiber composite material structural components, it is a known procedure to use the unidirectional layer as a flange that absorbs the normal forces from the flexural moments. In addition to the unidirectional laid scrim for the flanges, the fiber composite material structural component comprises the shear web that is made of a woven fabric or of a laid scrim, whereby in the case of a woven fabric, an undulation of the fibers is created by the weaving and this can lead to a lowering of the fiber-parallel compressive strength. In a laid scrim, which is held together by paper or vane stitching, the fibers ideally lie parallel and stretched.
In order to produce a preform with the three-dimensional shape of a curved fiber composite material structural component—on the basis of available prepregs, i.e. dry or optionally pre-impregnated fibers combined to form certain laid scrims—it is a known approach to use sewing techniques to produce individual preform parts with their final shape, i.e. three-dimensional individual preform parts that are dimensionally stable in three-dimensional space. These individual preform parts are sewed to form three-dimensional preforms. The sewing procedure here is three-dimensional.
As an alternative, so-called binder preforming is used with which, however, the placement of unidirectional tapes into the flanges is difficult.